Public safety agencies have a need for a communication network providing increased functionality, secure communication, and improved digital voice quality over a wider coverage area. One such network is defined by the Association of Public-Safety Communications Officials (APCO) Project 25 (P25). APCO P25 is an industry-wide effort to set the recommended voluntary standards of uniform digital two-way radio technology for public safety organizations. Key objectives of these standards include providing enhanced functionality with equipment and capabilities focused on public safety needs, improving spectrum efficiency, ensuring competition among multiple vendors through Open Systems Architecture, and allowing effective, efficient, and reliable intra-agency and inter-agency communications.
One important aspect of any wireless communication standard is providing a mechanism for signaling synchronization information to both receiving and transmitting radios. One such mechanism is defined in the APCO Project 25 standard, where an infrastructure device provides synchronization information through the outbound control channel (CCH) or voice channel (VCH). In certain situations, when the subscribers of a communication network such as a Time Division Multiple Access (TDMA) network, receive a call assignment before achieving synchronization with the VCH or when the CCH and VCH in the network are not synchronized with each other, the APCO P25 Phase 2 standard “PROJECT 25 Phase 2 Two-Slot Time Division Multiple Access Media Access Control Layer Protocol Specification-Trunked Voice Services” defines the use of Inter Slot Signaling Channels (ISCHs) to achieve synchronization. The ISCHs are present in the outbound transmissions sent by the infrastructure device over the VCH. The outbound transmission has an Ultraframe structure including a plurality of ISCHs. Although the standard defines the placement and structure of the ISCHs within the Ultraframe, it does not provide with any specific mechanism to synchronize to the Ultraframe structure.
Accordingly, there is a need for a method and apparatus for achieving synchronization in a wireless communication system.
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